Recently the synthesis and purification of a series of nitro-substituted cyclopenta-fused polycyclic aromatic hydrocarbons was reported. On the basis of expected charge distributions, these chemicals were predicted to be potent mutagens, but were found to be only weakly mutagenic for Salmonella. The report suggested that application of CASE, an artificial intelligence system recently developed in these laboratories, would also not predict the low mutagenicity of this group of chemicals. In the present report, it is shown that CASE, in fact, correctly predicts the low mutagenicity of nitro-substituted cyclopenta-fused polycyclic aromatic hydrocarbons.(Copyright (c) 1988 Elsevier Science Publishers B.V. Biomedical Division.)